Information storage devices are commonly used in environments that include vibration with vibration frequencies higher than 500 Hz. For example, sources of such vibration may include audio vibration from speakers driven by notebook personal computers, and/or cooling fans like those that are typically mounted in desktop or server computer systems. Unfortunately, some information storage devices (for example, hard disk drives) are especially sensitive to such vibration. For example, the operation of a hard disk drive depends upon the ability of a servo control system to precisely control the position of a read/write head relative to tracks of information on an adjacent spinning disk. However, the bandwidth of a typical servo control system limits its ability to correct positioning errors that fluctuate faster than 500 Hz. Indeed, some servo control systems used in hard disk drives may undesirably amplify, rather than correct or attenuate, position errors that fluctuate in the range of 1000-4000 Hz. Hence, it is important for manufacturers and customers to test the performance and/or robustness of hard disk drives and other devices under conditions that include such vibration.
Many contemporary electro-dynamic vibration testing apparatuses, also known as “shakers,” use a large motor to subject devices-under-test to vibration. For example, the voice coil motor may apply an oscillating force to a so-called slip table to which the device-under-test is mounted. Such shakers are typically expensive, requiring a motor that can develop large forces (e.g. 5000 lbf). Such shakers are also typically heavy, often having resonances in the 700-2000 Hz range. Bearings or rollers, designed to constrain the motion of these shakers, can contribute noise in the shaker acceleration. If required to accomplish vibration testing at a frequency in the range of its own resonances, such contemporary electro-dynamic shakers may also exhibit excessive harmonic distortion and/or off-axis accelerations (i.e. accelerations having an excessive component that is orthogonal to the nominal vibration direction). Although such large shakers may serve with adequate repeatability and linearity for certain conventional vibration tests, at higher frequencies and lower amplitudes harmonic distortion and unspecified off-axis vibration components often render such shakers inadequate. For example, if the desired amplitude of applied acceleration fluctuation is low (e.g. 0.2 g), in some cases the undesired off-axis component may even exceed the testing accelerations in the desired testing direction.
Thus, there is a need in the art for a vibration testing apparatus with reduced off-axis motion, noise, and/or harmonic distortion.